Human and entomological surveillance of West Nile fever, dengue and chikungunya in Veneto Region, Italy, 2010-2012

Abstract

Background: Since 2010 Veneto region (North-Eastern Italy) planned a special integrated surveillance of summer fevers to promptly identify cases of West Nile Fever (WNF), dengue (DENV) and chikungunya (CHIKV). The objectives of this study were (i) To increase the detection rate of imported CHIKV and DENV cases in travellers from endemic areas and promptly identify potential autochthonous cases.(ii) To detect autochthonous cases of WNF, besides those of West Nile Neuroinvasive Disease (WNND) that were already included in a national surveillance.

Methods: Human surveillance: a traveler who had returned within the previous 15 days from endemic countries, with fever >38°C, absence of leucocytosis (leukocyte count <10,000 μL), and absence of other obvious causes of fever, after ruling out malaria, was considered a possible case of CHIKV or DENV. A possible autochthonous case of WNF was defined as a patient with fever >38°C for <7 days, no recent travel history and absence of other obvious causes of fever. Entomologic surveillance: for West Nile (WNV) it was carried out from May through November placing CDC-CO2 traps in five provinces of Veneto Region, while for DENV and CHIKV it was also performed around residences of viremic cases.

Results: Human surveillance: between 2010 and 2012, 234 patients with fever after travelling were screened, of which 27 (11,5%) were found infected (24 with DENV and 3 with CHIKV). No autochthonous case of DENV or CHIKV was detected. Autochthonous patients screened for WNF were 408, and 24 (5,9%) were confirmed cases. Entomologic surveillance: the WNV was found in 10, 2 and 11 pools of Culex pipiens from 2010 to 2012 respectively, in sites of Rovigo, Verona, Venezia and Treviso provinces). No infected Aedes albopictus with DENV or CHIKV was found.

Conclusions: Veneto is the only Italian region reporting WNV human cases every year since 2008. WNV is likely to cause sporadic cases and unforeseeable outbreaks for decades. Including WNF in surveillance provides additional information and possibly an early alert system. Timely detection of DENV and CHIKV should prompt vector control measures to prevent local outbreaks.

Figure 1

Distribution of Ae. albopictus in Veneto region, according to the year of first detection.

Figure 2

Algorithm for detection of possible cases of West Nile Fever, Veneto Region, since 2011. N, no; Y, Yes; WNND, West Nile neuroinvasive disease; neg, negative; pos, positive.

Figure 3

Algorithm for management of possible cases of dengue and chikungunya, Veneto Region, since 2011. Neg, negative; pos, positive.

Figure 4

Maps showing the relative density of Cx. pipiens , the mosquito traps activated and positive for WNV and the municipalities with human cases (WNF, WNND and donors) in 2010, 2011 and 2012.

Figure 5

Relationship between vector index and human cases reported in the 15 days following the record of a WNV positive trap in 2012. The day of the onset of symptoms and the day of donation were considered for WNF/WNND and donors, respectively. When human cases are reported in the absence of mosquito positivity, a vector index = 0 in the closest trap in the previous two week has been reported.